The present invention relates to a process for the aromatization of hydrocarbons containing between 5 and 9 carbon atoms per molecule in the presence of a composite catalyst incorporating a MFI structure zeolite containing silicon, aluminium and at least one metal from the platinum family, to which are added at least one additional metal chosen from within the group constituted by tin, germanium, indium and lead. An amorphous matrix can be added to the catalyst for the shaping thereof.
Catalysts based on zeolites doped with gallium, zinc and platinum are known to be active and selective in the aromatization of propane and butane. Conventionally hydrocarbons with more than 6 carbon atoms per molecule are transformed into aromatics by catalytic reforming using catalysts of the platinum-doped acid alumina type platinum to which can be added tin, rhenium, etc. However, these reforming catalysts do not have good performance characteristics for the aromatization of hydrocarbons with 5 carbon atoms per molecule and to a lesser extent for those with 6 carbon atoms per molecule. Thus, there is considerable interest in finding high performance catalysts for the aromatization of hydrocarbon-rich fractions of the C.sub.5 -C.sub.6 type.
The aromatization reaction of hydrocarbons with more than 5 carbon atoms per molecule in the presence of zeolitic catalysts has already formed the subject matter of several patents and publications. Several MFI zeolite-based catalytic systems are claimed and these systems can be distinguished by the additives contained therein. A distinction can be made between:
i) systems containing gallium (G. Berti, J. Moore, L. Salusinszki, D. Seddon, Aust. J. Chem., 42, p 2095, 1989) PA1 ii) systems containing zinc (O. Anunziata, O. Orio, L. Pierella, M. Aguirre, React. kin. Catal. Lett., 39(1), 75, 1989; J. Kanai, N. Kawata, J. Catal., 114, 284, 1988). PA1 (a) The MFI zeolite is impregnated with an aqueous solution of a compound of a metal chosen from within the group constituted by tin, germanium, indium and lead. PA1 (b) The product obtained in stage (a) is dried. PA1 (c) The product obtained in stage (b) is calcined. PA1 (d) The product obtained in stage (c) is impregnated with a platinum acetyl acetonate solution in toluene. PA1 (e) The product obtained in stage (d) is dried. PA1 (f) The product obtained in stage (e) is calcined. PA1 (g) The product obtained in stage (f) is reduced under a hydrogen stream. PA1 (a) The MFI zeolite is impregnated with an aqueous solution of a compound of a metal chosen from within the group constituted by tin, indium, germanium and lead. PA1 (b) The product obtained in stage (a) is dried. PA1 (c) The product obtained in stage (b) is calcined. PA1 (d) The product obtained in stage (c) is impregnated with an ammoniacal tetraammine platinum chloride solution. PA1 (e) The product obtained in stage (d) is dried. PA1 (f) The product obtained in stage (e) is calcined. PA1 (g) The product obtained in stage (f) is reduced under a hydrogen stream. PA1 (a) The MFI zeolite is impregnated with an ammoniacal tetraammine platinum chloride solution. PA1 (b) The product obtained in stage (a) is dried. PA1 (c) The product obtained in stage (b) is calcined. PA1 (d) The product obtained in stage (c) is reduced under a hydrogen stream. PA1 (e) The product obtained in stage (d) is contacted with a hydrocarbon solvent and with said organic compound of metal M, e.g. by immersing the mass in a hydrocarbon solvent already containing the organic compound or by immersing the mass in a hydrocarbon solvent and then injecting into the mixture obtained a solution of the organic compound of said metal M in a hydrocarbon solvent and e.g. that in which said mass has been immersed. PA1 (f) The product obtained in stage (e) is reduced under a hydrogen stream.
However, all these systems suffer from an important defect, namely a high methane selectivity. In order to improve the performance characteristics of these catalytic systems a number of solutions have been proposed, such as the addition of platinum (Z. Jin, Y. Makino, A. Miyamoto, T. Inui, Chem. Express, 2 p 515, 1987). The use of a non-acid MFI zeolite with various metallic elements has also been claimed (Y. Chen. et al. WO 8904818).
Moreover, in French patent application 90/06557, the Applicant claims a catalyst incorporating on the one hand a MFI zeolite and on the other a generally amorphous matrix or support, on which is deposited a noble metal from the platinum group and at least one additional metal chosen from within the group constituted by tin, germanium, indium and lead, said support also containing at least one alkali metal and at least one alkaline earth metal chosen from within the group constituted by lithium, sodium, potassium, rubidium, cesium, barium, calcium, beryllium, magnesium and strontium. This catalyst is used for the aromatization of hydrocarbons with 2 to 4 carbon atoms per molecule.